1. Field of the Invention
The present invention relates to a thermistor device having a positive thermal coefficient. More particularly, the present invention pertains to a PTC (positive thermal coefficient) thermistor device including a disc-shaped PTC element held in a casing provided with plug elements for external connection.
2. Description of the Prior Art
A PTC thermistor device has been widely used in a starting circuit for induction motors as a motor starting relay. Examples of such thermistor are disclosed by the Japanese utility model publication 58-34722 and the Japanese patent publication 63-18817. FIGS. 19 through 21 show a PTC thermistor device of this type. Referring to FIG. 20, it will be noted that the thermistor device has a PTC element 1 comprising a circular disc body 101 of a PTC material and a pair of electrodes 102 and 103 provided at the opposite sides of the body 101. The PTC element 1 is located in a casing 2 of an electrically insulative material such as a plastic material. The casing is formed with an internal space 21 for housing the PTC element 1. In order to support the PTC element 1 in the casing 2, there is provided an insulative plate 5 which has a circular opening 51 to which the PTC element 1 is fitted. The casing is formed at the inner surface with a groove 22 for receiving the periphery of the insulative plate 5. Thus, the PTC element 1 is loosely supported by the casing 2 by having the periphery of the insulative plate engaged with the groove 51 in the casing 2.
At the opposite sides of the PTC element 1, there are provided terminal assemblies 3A and 3B each of which includes a contact 31 and a socket receptacle 32. Each of the contact 31 is attached to the inner wall of the casing 2 at an intermediate portion and formed with two resilient leg portions which are in resilient contact with one of the electrodes 102 and 103. The contacts 31 are electrically connected with respective ones of the socket receptacles 32. As shown in FIGS. 20 and 21, the socket receptacles 32 are arranged in parallel with each other with one ends exposed outside at one surface 25 of the casing 2 to thereby form a socket.
There are also provided a pair of plug legs 33 which are connected with respective ones of the socket receptacles 32 and project outwardly from the side of the casing 2 opposite to the side where the socket receptacles 32 are exposed outside. As shown in FIG. 19, the casing is formed with a semi-cylindrical portion having a pair of opposite side surfaces 26 and 27 and a semi-cylindrical outer surface 28 for accommodating the PTC element 1 of a circular disc shape. The plug legs 33 extend outwardly substantially along the side surfaces 26 and 27 and have outer end portions 331 extending beyond the outer surface 28 of the semi-cylindrical portion of the casing 2.
As shown in FIG. 21, the thermistor device of the aforementioned structure is attached to a motor M by inserting terminal plugs T on the motor M into the socket receptacles 32 of the terminal assemblies 3A and 3B with the plug legs 33 projecting toward the other side. The plug legs 33 are connected with a socket for connecting the thermistor device with another part such as a capacitor.
In this type of thermistor device, it has been experienced that in mounting the device on the motor by having the socket receptacles 33 engaged with the plugs T on the motor M fingers of a workman are apt to slip on the outer surface of a round configuration to make the assembling work difficult. It should further be noted that the plug legs 33 projecting beyond the outer surface 28 of the casing 2 are very often deformed or contaminated by being touched by fingers of a workman. The contamination of the plug legs may cause corrosion and reliability of electrical contact may be destroyed.
It should further be noted that in the thermistor device of this type dusts are apt to accumulate on the round outer surface 28 of the casing 2. Such accumulation of dusts may cause a decrease in insulative property between the plug legs 33 because the dusts on the surface 28 have water absorbing property. As the result, there may be produced a surface discharge along the outer surface 28 of the casing 2.